Argon is typically produced through the cryogenic rectification of the air conducted in an air separation unit. The air separation unit consists of compressors to compress the air, a purification to purify the air by removal of higher boiling impurities, a main heat exchanger to cool the air and a distillation column system to rectify the compressed, purified and cooled air and thereby produce an argon product.
The distillation column system can be provided with a double column unit having a higher pressure column and a lower pressure column operatively associated in a heat transfer relationship by a condenser reboiler. The higher pressure column, so designated because it operates at a higher pressure than the lower pressure column, distills the incoming air to produce a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms also known as kettle liquid. A stream of the crude liquid oxygen column bottoms is in turn further refined in the lower pressure column to produce an oxygen-rich liquid column bottoms and a nitrogen-rich vapor column overhead. Oxygen-rich and nitrogen-rich product streams can be heated in the main heat exchanger to help cool the incoming compressed and purified air. An argon and oxygen containing vapor stream, removed from the lower pressure column near at a point of maximum argon concentration, serves as a crude argon feed stream to an argon column to separate the argon from the oxygen and thereby to produce an argon-rich vapor column overhead. A heat exchanger is connected to the argon column to condense a stream of the argon-rich vapor column overhead to produce reflux to the argon column and a liquid argon product. Depending upon the number of stages of separation contained in the argon column, the liquid argon product may be directly taken or further refined as necessary with a catalytic unit to remove oxygen and another distillation column to separate out the nitrogen contained in the argon.
Typically, the heat exchanger used in condensing the argon-rich vapor column overhead is a thermosiphon type of heat exchanger in which a heat exchange core is situated within a shell. The crude liquid oxygen is introduced into the shell and is partially vaporized through indirect heat exchange with the argon-rich vapor passing through condensation passages of the heat exchange core. The argon-rich vapor is condensed and residual liquid within the shell due to the partial vaporization of the crude liquid oxygen is drawn through open vaporization passages of heat exchange core through the thermosiphon effect. The vapor and liquid phases can be separately introduced into the lower pressure column for further refinement of the crude liquid oxygen. An oxygen containing column bottoms produced in the argon column as a result of the separation of argon and oxygen is also returned to the lower pressure column. When a single core does not have the necessary surface area, a series of cores can be positioned within the shell.
A more cost effective method of condensing argon-rich vapor is to use once-through heat exchangers in which the crude liquid oxygen and argon-rich vapor are separately introduced into adjacent boiling and condensation passages. While this type of arrangement uses less components than a thermosiphon arrangement, where the heat exchange duty needs to be divided into two or more heat exchangers, dry out becomes a significant problem because high boiling temperature hydrocarbon components can freeze out and concentrate leading to flammability hazards. This problem arises because the heat exchangers are sited at a sufficiently high level as compared to the higher pressure column that the loss of head results in the flashing of the liquid into vapor and therefore, control of the flow to ensure that sufficient crude liquid oxygen is introduced into each of the heat exchangers is problematical.